In controlling various oil field equipment, such as blowout preventers, production trees and other hydraulically actuated equipment, it is customary to provide a hydraulic supply at a high pressure, for example 3,000 psi. However, the equipment works at a pressure lower than the supply pressure and it is necessary to reduce and regulate the pressure to the desired working pressure for the equipment. For example, in present subsea control systems, a hydraulic piloted pressure reducing and regulating valve, Part No. 1112-4100, shown on page 201 of the General Catalog 1982-1983 of Koomey, Inc. illustrates a conventional hydraulic piloted pressure regulating valve which has an inlet pressure of 3,000 psi and provides a regulated outlet pressure of 300 to 1,500 psi. In such a valve a remote pilot signal pressure is connected to the regulator valve to provide a remote means for adjusting the regulated pressure to a control piston which then moves in response to flow demands on the valve.
The problem with the present hydraulic piloted pressure reducing and regulating valves is that if the control system fails thereby causing a loss of the pilot signal this is equivalent to issuing a command signalling the regulating valve to vent off the regulated pressure to zero. This means that the equipment being operated by the outlet supply pressure from the valve becomes inoperative. In the case of safety equipment such as oil well blowout preventers this could lead to a disaster.
The present invention is directed to a hydraulic piloted pressure reducing and regulating valve which includes a fail safe means for holding the valve at the last selected operating pressure in the event of the loss of control signals. Thus, even though the valve may be unable to change its regulated pressure, the regulated pressure will not drop to zero, but will be maintained at its last pressure setting. Therefore, the controlled equipment will still be operable.